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Soil Health Series: Soil Chemical Quality Indicators

Soil Health Series: Soil Chemical Quality Indicators

Soil health emphasises the soil biological community as a key driver of soils capacity to deliver multiple functions. Researcher Karen Daly tells us about chemical indicators that can be used to assess the health of our soils.

How do we define soil health?

Soil quality has been described as the soil’s ability to provide a range of different services through its capacity to perform soil functions under changing management and climatic conditions. Recently this term has been replaced by ‘soil health’, which emphasises the soil biological community as a key driver of soils capacity to deliver multiple functions. Soil health has been defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans. In light of this, developing knowledge and understanding of factors affecting soil health and monitoring the trends in soil health over time is required to better manage and protect our agricultural soils for future generations.

What chemical indicators can we used to assess the health of our soils?

Researchers at Teagasc Johnstown Castle (Karen Daly and Giulia Bondi) describe soil chemical indicators that influence nutrient supply and storage as part of the SQUARE project. Nutrients such as nitrogen (N) and phosphorus (P) are essential for crop growth and animal health and these nutrients can be stored in soil and made available when crops need them. A healthy soil will have the ability to immobilise (store) and mineralise (supply) nutrients and this function relies on a number of soil properties to be in good working order. Soil organic matter is often called the engine room inside the soil matrix, and this is where many of the soil chemical and biological reactions occur. A healthy amount of soil organic matter is essential for many of the processes that control nutrient supply and storage in soils. If soil organic matter is depleted or reduced, this inhibits the soils ability to provide soluble forms of nutrients, and more importantly our ability to store and sequester carbon. As we move towards low emission agriculture, our soil carbon stocks will be hugely important to protect and enhance. Also, for nutrient supply to function at full capacity other soil chemical conditions must be met, for example, soil pH provides the right environment for nutrients to become soluble and for reactions on clay surfaces to happen. The moisture content of soil is also important when it comes to providing nutrients in the soil solution for diffusion into plant roots, and this links back to soil structure, where soil drainage class plays an important role, and the amount and type of clays and organic matter provide surfaces for nutrient reactions to happen.

Tools to be deployed as part of a national monitoring programme

Deciding exactly which soil properties should be monitored will need to be considered very carefully. We need to decide which soil properties can be directly related to soil health and which soil functions we need to protect. For soil chemical quality, we can clearly see a role for properties such as soil organic matter and pH in both chemical and biological functions in soils. Monitoring changes in these both of these properties over time and with changes in land use change will be important. Available nutrients such as N and P are dynamic, however, identifying some of the static soil properties that influence their behaviour in soil such as soil texture, drainage class, organic matter, and geochemical properties, can give us an idea of what kind of nutrient dynamics to expect in a particular area or soil type. Likewise for soil carbon, a monitoring network of our existing stocks and ability to sequester carbon will be critical in developing tools to mitigate climate impacts from agriculture.

What next?

Going forward we will need long term monitoring of soils to detect changes in soil health over time. Such a network will act as an early warning system before problems arise on farms. Simultaneously, we need practical management solutions for protecting the health and quality of our agricultural soil or for remediating soils which have been previously damaged. This knowledge can be integrated with knowledge transfer services to provide advice to farmers and farm advisors. We have made a good start but we have a long way to go!